Electrical machines, such as motors and generators having a rotor with permanent magnets are known. They are generally deemed to be reliable and require less maintenance than other generator topologies.
Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft either directly drives the generator rotor (“directly driven”) or through the use of a gearbox. Particularly for offshore wind turbines, direct drive systems employing permanent magnets are usually chosen.
Such a direct drive wind turbine generator may have e.g. a diameter of 6-8 meters and a length of e.g. 2-3 meters. Hundreds of magnets may need to be attached, e.g. by screwing or gluing to the circumference of the rotor. It will be clear that this manufacturing process can be cumbersome.
Furthermore, if one or more of the magnets is damaged and needs to be replaced, the access to these magnets can be complicated (especially with large size generators), such that maintenance can become very expensive.
This size and type of generator however is not limited to offshore applications, and not even to the field of wind turbines only. Generators of considerable dimensions that may suffer from the same problems and/or have the same complications may also be found e.g. in steam turbines and water turbines. Also relatively large permanent magnet motors may have the same or similar problems.
Examples of permanent magnet rotors that aim at dealing with the aforementioned problems may comprise a rotor rim and a plurality of permanent magnet modules arranged on the outer or inner circumference of the rotor rim. The permanent magnet modules may extend generally along an axial direction and may be of substantially constant axial-cross section. The permanent magnet modules may comprise a base adapted to be fixed to the rim of the generator rotor and a central magnet structure. The central magnet structure may comprise one or more permanent magnets, and a central magnet support structure acting as a flux concentrator. The central magnet support structure may comprise a plurality of radial holes. The base may also comprise a plurality of radial holes aligned with the radial holes of the central magnet support structure. Bolts or screws inserted in the radial holes may be used to fix the central magnet structure to the base. Once the permanent magnet module is assembled, a first surface of each magnet should be in contact with the central magnet support structure and a second surface with the base.
However, a problem may arise when such a permanent magnet module is assembled. As the bolts are tightened, the central magnet support structure exerts a non-uniform pressure on the lateral sides or wings of the base. As a result the base of the permanent magnet module may deform and stress concentration points or areas may appear. When the base is deformed, at least one side of the permanent magnets may become detached. Consequently, when the rim rotates, during operation, the permanent magnets may vibrate. Such vibrations may increase the noise levels of the permanent magnet rotor, may accelerate wear of the central magnet structure and may reduce the magnetic flux. Consequently, the effectiveness of the flux concentrator is compromised.
It would be desirable to provide a permanent magnet rotor in which the above drawbacks are at least partially solved.